The present invention relates to a radiographic imaging system whereby a subject is irradiated with radiation and whereby the radiation that has penetrated the subject is detected and converted into an electric signal by a radiation detector to produce a radiographic image based upon the obtained electric signal. The present invention relates in particular to a radiographic imaging system whereby an image of an imaging region longer than the imaging surface of the radiation detector is produced.
Radiographic imaging systems are used in a variety of fields such as medicine where they are used to produce diagnostic images for medical use and industries where they are used to conduct nondestructive tests. Presently, some radiographic imaging systems use a flat panel detector (FPD) that converts radiation into an electric signal as a radiation detector for detecting the radiation that has penetrated a subject (e.g., X-ray, alpha ray, beta ray, gamma ray, electron beam, and ultraviolet ray).
In a radiographic imaging system using an FPD, a subject is irradiated with radiation emitted from a radiation source, whereupon the FPD converts the radiation that has penetrated the subject into an electric signal and reads out an electric signal corresponding to image data of the subject to produce a radiographic image.
Presently, a typical FPD has a size of only about 43 cm×43 cm. Thus, it is impossible to take a radiographic image of a long region such as the whole region of a spine (the whole spine) and the whole region of lower extremities (the whole lower extremities) of a subject (examinee).
To take a radiographic image of such a long region, radiographic imaging systems using an FPD moves the FPD in the body axis direction while effecting a plurality of exposures, i.e., taking a plurality of short images to achieve imaging of a long region, as described in JP 2004-358254 A, JP 2005-270277 A, and JP 2006-500126 A, which is referred to as long region imaging.
Specifically, when producing a long image, the number of times images are taken and the positions in which images are taken are determined according to the imaging region to be covered and the size of the FPD used, and the FPD and the radiation exposure field are moved in the body axis direction according to the determined imaging positions to take short images in different regions a plurality of times (a predetermined number of times images are to be taken), thereby producing an image of a long region covering the whole spine or the whole lower extremities. In long region imaging, short radiographic images thus taken are combined to obtain a long radiographic image of the whole spine or the whole lower extremities.
In a radiographic imaging system using the FPD, a part of the electric charge corresponding to the image data remains in the FPD even after an electric signal corresponding to the image data has been read from the FPD. When another image is taken with the remaining electric charge, the remaining electric charge inside the FPD is superimposed upon the next radiographic image as residual image, resulting in a radiographic image affected by the residual image.
Such a residual image decreases with time. Accordingly, in a case of normal imaging where an image of a whole region of the subject is taken by a single exposure, effects of a residual image does not pose a problem since a sufficient time interval is provided between an imaging and a subsequent imaging.
In long region imaging, however, about two to five images are taken successively at very short intervals as compared with normal imaging in such a manner that a short image is taken, followed by displacement of the FPD, again a next short image is taken, followed by displacement of the FPD, repeating this process. Accordingly, the residual image of the previous short image has not decreased sufficiently when the next short image is taken. This causes a problem that a short image is affected by the residual image of the previous short image, precluding a proper diagnosis.